1. Field of the Invention
The present invention relates to a process for grinding calcium carbonate in aqueous media in the presence of a water-soluble grinding agent in a quantity far below that presently used in the art, which nevertheless leads to fine suspensions with a concentration of dry substance of at least 70% by weight and with a particle size of two micrometers at the most.
2. Description of the Background
It is well known that inorganic substances such as calcium carbonate, sulfate, and silicate, as well as titanium dioxide can be used in for the preparation of industrial products which in turn are used in the paint, paper-coating, rubber-extender, synthetic resin and the like fields. However, since these inorganic substances do not have a natural lamellar or laminar structure to facilitate their disintegration, as is the case with certain substances such as the aluminum silicates commonly known by the name of kaolin, one skilled in the art has to transform them by grinding to a fine particle size aqueous suspension whose constituent particles have the smallest possible dimension, i.e., less than several microns, in order to be able to use them in the pigment field.
The number of publications in the specialized literature in this field reveals the importance and complexity of grinding inorganic substances in aqueous media to obtain a partially refined quality that permits application in pigments. This is why it is well known in the particular case of paper coating that coating compositions formed of inorganic pigments such as kaolin, calcium carbonate, calcium sulfate, and titanium dioxide in suspension in water also contains binders and dispersing agents as well as other additives such as thickeners and coloring agents. It is desirable for such coating compositions to have a low and stable viscosity for the duration of the coating operations, to facilitate handling and application of the compositions and at the same time to have a content of inorganic material as high as possible to reduce the amount of heat energy necessary to remove the aqueous fraction of the coating composition by drying. Such an ideal suspension combining all of these fundamental qualities would solve for the person skilled in the art the well-known problems of grinding, storage, transport from the place of production to the place of application, and finally of pumping during its use.
It has been found that the grinding techniques for inorganic substances in aqueous media have led to suspensions that are unstable with time, because of the sedimentation effects of the inorganic substances and the increase of viscosity of the aqueous medium. Therefore, those skilled in the art have become resigned in the processes related to the older techniques of obtaining pigment substances by grinding, to accomplish the grinding of the inorganic substance in aqueous suspension by one or more successive operations, and then to drying and classifying the ground substance while eliminating the insufficiently small particles, and separating the inorganic particles with the desired pigment dimensions to obtain a fine inorganic powder of low water content. Thus, this fine powder, intended for pigment applications, is readily transportable from its place of production to the place of use where it is again placed in suspension in water to be used as a pigment product.
Consequently, the fact that the inorganic pigment cannot be kept in the form of an aqueous suspension between the grinding operations and application has induced those of skill in the art to proceed with research in this field, which has consisted of carrying out the grinding of inorganic substances in aqueous suspension to provide, after grinding, a pigment suspension of low viscosity that is stable with time. Thus, for example, French Patent No. 1 506 724 suggests a process for preparing an aqueous suspension of calcium carbonate by grinding, which is stable with time. The method consists of forming an aqueous suspension by agitation which contains 25 to 50% by weight of calcium carbonate, and then grinding the aqueous material with a suitable grinding substance in the presence of a dispersing agent. This dispersing agent is an acrylic polymer soluble in water that is added to the grinding medium in an amount of 0.2 to 0.4% by weight of the calcium carbonate present. In spite of the undeniable benefit of having an inorganic pigment suspension which exhibits good time stability, it appears that such a suspension has drawbacks that can be considered to be significant by one skilled in the art. This is the case, for example, with the content of dry substance of such a suspension intended for grinding, which necessarily has to be between 25% and 50% by weight, and preferably equal to about 40% by weight, because if the concentration is below 25%, the process described is of little economic interest because of its low productivity, and if the initial concentration of dry substance is above 50%, the efficiency of the grinding process decreases because of the large increase of the viscosity of the medium which becomes an obstacle to the grinding steps, which produces a suspension of coarse sized particles. Thus, when the concentration of dry substance in the suspension intended for grinding is chosen in the range of 25% to 50% by weight, the particle size of the calcium carbonate obtained after a very long period of grinding in the presence of the dispersing agent can be considered to be favorable for pigment applications, since 95% of the particles have a maximum dimension smaller than two microns.
The fact that an aqueous suspension of inorganic substances cannot be ground at a concentration above 50%, because of the great increase of viscosity, has prompted those skilled in the art to find another method. Consequently, processes have been suggested for forming aqueous suspensions of inorganic substances, not including grinding, which have a high content of dry substance. French Patent No. 1 562 326, for example, discloses a process for preparing an aqueous suspension of inorganic substances. The objective of this process is to prepare a highly concentrated aqueous suspension of dry substance that is sufficiently stable so that it can be transported in this form from its point of manufacture to its point of use. The process proposed consists of forming an aqueous suspension of inorganic substances containing from 70 to 85% by weight of dry substance, of which at least 99% by weight of the initial particles have a dimension below or equal to 50 microns, and then of agitating this suspension in the presence of a dispersing agent, which can be a sodium or potassium salt of a polyphosphoric acid, a polyacrylic acid, a polysilicic acid, etc., obtained by complete neutralization of the corresponding acid with sodium or potassium hydroxide. This dispersing agent is then introduced at a level of 0.05% to 0.5% by weight relative to the weight of dry substance contained in the aforesaid suspension. It is stated that the content of dry substance in the suspension should not be above 85% by weight because of the great increase of its viscosity, and cannot be below 70% by weight without causing undesirable sedimentation. However, this technique is not entirely satisfactory.
Another solution to the problem requires grinding, in the presence of a dispersing agent, of an aqueous suspension of calcium carbonate which has too low a concentration of dry substance to be of interest, although it has the advantage of producing a pigment suspension of low viscosity and small particle size, providing up to 95% of particles with sizes below two microns.
Still another solution to the problem requires the preparation of an aqueous suspension with a high content of dry substance, containing 70% to 85% by weight of inorganic substances, 99% of the initial particles of which have a dimension below or equal to 50 microns. A dispersing agent is introduced into the medium that stabilizes these particles.
A number of efforts have been made to make the grinding agents derived from the chemistry of carboxylic acids more effective for obtaining concentrated suspensions of finely divided calcium carbonate. Thus, French Patent Number 2 539 137 describes a grinding agent based on acrylic polymers and/or copolymers whose molecular weight has been properly selected so that it brackets specific viscosities ranging from 0.50-0.60, whose neutralization is accomplished with at least one neutralizing agent containing a monovalent cation and at least one neutralizing agent containing a polyvalent cation.
Moreover, European Patent 0 108 842 proposes an improvement of the grinding agent in which a sulfonated comonomer such as acrylamidopropanesulfonic acid is introduced into the molecule of the carboxylic polymer. However, in all cases the suspensions of calcium carbonate with high concentration and very small particle size are obtained with high energy consumption. This causes a temperature increase of the medium that exceeds 100.degree. C. most of the time, and high consumption of the order of 1% of the grinding agent in the dry state relative to the weight of dry calcium carbonate, when it is desired to obtain more than 60% of particles below 1 micron and a concentration of calcium carbonate greater than 70%. The use of excessively large quantities of grinding agent, which are nevertheless necessary up to now to obtain these fine, concentrated suspensions with acceptable rheology and stability for paper coating applications, has several major drawbacks which include the following:
i) A first drawback is found in the paper industry when "brokes" are recycled, because too high a supply of anionic polyacrylate from recycling causes the at least partial inhibition of the cationic retention agents used and interferes with sheet formation.
ii) Another drawback is found in the fact that these agents which can be found, in part, in waste water impede its clarification and contribute to increasing the chemical oxygen demand (COD) of water, which is unacceptable to water treatment authorities,
iii) Still another drawback that arises is the need to protect people against toxic products. That is, paper products containing the conventional inorganic substances, which come into contact with food, must contain limited amounts of such agents, amounts that are not achievable with the present grinding processes and grinding agents.
iv) Finally, the excess grinding agent constitutes a useless waste.
A need therefore continues to exist for an improved method of forming ground pigment suspensions.